Hong Kong's Chief Executive has told Sky News the alleged abduction of a British bookseller from the territory would be unacceptable if proven, and that his government is placing "a great deal of emphasis" on the case.

CY Leung said Hong Kong police were investigating the disappearance of Lee Bo, who has been missing since 30 December.

Mr Lee is the fifth Hong Kong-based bookseller to vanish mysteriously in recent months in a case that raises serious questions about the protection of freedoms promised to the territory during its handover from British rule.

Local activists fear the men are being held by mainland Chinese security forces after publishing works critical of the Communist party leadership.

Asked whether he had raised the case with authorities in Beijing, CY Leung told me: "The short answer is I have."

"There have been reports of mainland authorities arresting the person, or abducting the person in Hong Kong, I said that if that was the case that would not be acceptable."

The five men are all linked to the Mighty Current publishing house - an outlet known for its provocative works about Communist Party leaders that are banned on the Chinese mainland, but legal in Hong Kong.

They are rumoured to have been working on a new book about the private life of President Xi Jinping.

Writer and publisher Gui Minhai was the first to disappear.

Last seen at his holiday home in Pattaya, Thailand, he had emailed printers on 15 October telling them he would send material for a new book.

The following week the publishing company's general manager Lui Bo, business manager Cheung Ji-ping, and bookstore manager Lam Wing-kei also went missing, all whilst visiting family, separately, in mainland China.

Their colleague, 65-year-old Lee Bo, also known as Paul Lee, raised the alarm.

At the time he said he felt safe in Hong Kong, where mainland Chinese security agents have no jurisdiction, but on 30 December, after visiting the company's warehouse, he too disappeared.

British officials in China have made urgent enquiries about his whereabouts, but have yet to locate him.

Foreign Secretary Philip Hammond said during a visit to Beijing that a cross-border abduction would be an "egregious breach" of the "one country, two systems" agreement meant to protect the rights of Hong Kong citizens for 50 years from its handover.

Some of the men have been in sporadic contact with family members by phone.

A few days after he went missing, Lee Bo called his wife, apparently from Shenzhen, a city just across the border to the north in mainland China.

He told her he was "assisting an investigation" and she should not make his case widely known.

But the call raised as many questions as it answered - he spoke in Mandarin, the dialect of the mainland, rather than his usual Cantonese, and he hadn't taken his travel permit with him - the document he should have used to cross the border if he really had gone there of his own free will.

She later withdrew her request for help from police after a handwritten letter was faxed to one of his friends saying he had gone to the mainland "by his own methods" and would be staying there for some time.

Some Hong Kong politicians have openly suggested that the letter may have been written under duress.

Human rights campaigners say mainland police often pressure the families of those detained to minimise publicity around the case, and CY Leung said the police investigation would continue.

Agnes Chow, a 19-year-old student activist, posted a video appeal about the disappearances online asking for international help and accusing Beijing of “political suppression” - it has been viewed close to a million times so far.

Titled "An Urgent Cry from Hong Kong" she adapted the famous words of German pastor Martin Niemoller.

"First they came for the activists, and I did not speak out because I was not an activist," she said.

"Then they came for the journalists, and I did not speak out because I was not a journalist.

"Then they came for the bookseller, and I did not speak out because I was not a bookseller.

"Then they came for me and there was no-one left to speak for me."

She urged foreign governments to speak out - and to challenge the leadership in Beijing about the case.

"All the governments should speak up and should help Hong Kong people to protect our basic human rights," she said.

"It's not only about democracy or freedom, it's about our own personal safety."

China's government has made no official comment on the case so far, but Foreign Minister Wang Yi has said Lee Bo is "first and foremost a Chinese citizen".

The Communist party-controlled Global Times newspaper published an editorial accusing the bookshop of spreading "evil influence" to the mainland, and said an investigation would be "not only reasonable, it also conforms with Chinese law".